CN114153135A - Locking method of cesium beam atomic clock - Google Patents

Abstract

The invention discloses a locking method of a cesium beam atomic clock, which comprises the following steps: step 1, microwave frequency modulation: modulating the microwave frequency; step 2, microwave frequency scanning: inputting microwaves into a cesium-beam tube to obtain atomic signals; step 3, frequency error signal demodulation: demodulating the atomic signal with the frequency modulated signal and low pass filtering to produce a frequency error signal e_ω(t); setting the microwave frequency at e_ω(t) at the maximum, the microwave power b is changed and an error signal e is recorded_ω(t) microwave power b at maximum₁(ii) a Step 4, microwave power modulation: at microwave power b₁Modulating the microwave power near the point; step 5, microwave frequency-power combined locking: for error signal e_ω(t) carrying out proportional-integral-derivative (PID) processing and feeding back to a microwave frequency setting end to realize frequency locking; to e_ωAnd (t) demodulating and low-pass filtering by using the power modulation signal to obtain a microwave power error signal, and carrying out PID (proportion integration differentiation) processing on the microwave power error signal and feeding back the microwave power error signal to a microwave power control end to realize power closed-loop locking.

Description

A locking method for cesium beam atomic clock

technical field

The invention relates to the field of atomic frequency standards, in particular to a locking method for a cesium beam atomic clock.

Background technique

An atomic clock is a timekeeping device that uses the transitions of atoms as a standard. It is currently the most accurate time and frequency standard, so its application range is extremely wide: from precise basic scientific measurements, such as the determination of physical constants, theoretical physics verification, to engineering applications that directly serve people's daily production and life, such as global navigation satellites system, etc. The cesium beam atomic clock has the characteristics of high accuracy and good long-term stability. It is the core equipment for establishing and maintaining a high-precision and high-stability punctuality system. application.

The long-term frequency stability of cesium beam atomic clocks is affected by various frequency shift factors, among which one of the main frequency shifts is the frequency shift of microwave power. In order to avoid the long-term drift of the output frequency of the atomic clock caused by the drift of the microwave power, it is usually necessary to jointly lock the microwave power and frequency.

The microwave power locking method of the existing cesium beam atomic clock is mainly to slowly modulate the microwave power, and directly demodulate the output signal of the cesium beam tube to generate an error signal and feed it back to the microwave power, so as to lock the microwave power to the cesium beam. The maximum output signal of the tube. This approach has two major drawbacks:

First, ideally, the relationship between the microwave power and the amplitude of the output signal of the cesium beam tube is shown in Figure 2, but in fact, due to the influence of the adjacent transition lines under high microwave power, the signal amplitude will increase when the microwave power increases. large, as shown in Figure 2, so the microwave power loss may occur when the traditional method is used for locking;

Second, the stability of the cesium atomic clock is positively correlated with the amplitude of the frequency-locking error signal. Usually, the maximum value of the signal and the maximum value of the error signal are not at the same microwave power point, so it is difficult for the traditional method to achieve the best performance index of atomic clock stability;

Therefore, the present invention proposes a new locking method for cesium beam atomic clock, which can stably lock the microwave power at the maximum point of the error signal, and can obtain better stability index and better robustness than the traditional scheme. Awesome.

SUMMARY OF THE INVENTION

For realizing the purpose of the present invention, adopt following technical scheme to realize:

A method for locking a cesium beam atomic clock, comprising the following steps: step 1. modulating the microwave frequency; step 2. scanning the microwave frequency to obtain a frequency error signal of frequency locking; step 3. scanning the microwave power; step 4. modulating the microwave power ; Step 5. Microwave frequency-power joint locking.

The locking method of the cesium beam atomic clock, wherein step 1 includes:

Let the original microwave signal be y(t)=bcos(ω ₀ t), where b is the amplitude of the microwave signal, ω ₀ is the atomic resonance frequency, and the modulated microwave signal is expressed as:

为微波频率调制幅度为ω_m，调制周期为T_ω，均值为0的频率调制信号；采用方波进行微波频率调制，即：in,

is a frequency modulation signal with a microwave frequency modulation amplitude of ω _m , a modulation period of T _ω , and an average value of 0; a square wave is used for microwave frequency modulation, namely:

为10+2Hz量级。frequency of the frequency modulated signal

It is of the order of 10+2Hz.

The locking method of the cesium beam atomic clock, wherein step 2 comprises:

解调产生用于频率锁定的频率误差信号e_ω(t)：2.1 Feed the microwave signal modulated in step 1 into the cesium beam tube to obtain the atomic photodetection signal r(t), and modulate the square wave with the frequency

Demodulation produces a frequency error signal e _ω (t) for frequency locking:

2.2 Fix the microwave power b, change the microwave frequency near the resonance point ω ₀ , find the maximum value of the frequency error signal, and record the microwave frequency as ω ₁ at this time.

The locking method of the cesium beam atomic clock, wherein step 3 comprises:

The microwave frequency is set at ω ₁ , and the microwave power b is changed to obtain the microwave power b ₁ when the frequency error signal e _ω (t) takes the maximum value.

The locking method of the cesium beam atomic clock, wherein step 4 includes: modulating the microwave power near the microwave power b ₁ point, and the modulated signal is expressed as:

为微波功率调制幅度为b_m，调制周期为T_b，均值为0的功率调制信号，采用方波进行微波功率调制，即：in,

is a power modulation signal with a microwave power modulation amplitude of b _m , a modulation period of T _b , and an average value of 0, using a square wave for microwave power modulation, namely:

为10^-2Hz量级。

on the order of ^10-2 Hz.

The locking method of the cesium beam atomic clock, wherein step 5 comprises:

5.1 Perform proportional-integral-derivative processing on the frequency error signal e _ω (t) to obtain the frequency control signal c _ω (t), namely:

In the complex frequency domain, it can be expressed as:

Wherein K _ωp , K _ωi , K _ωd are PID gains, and the frequency control signal c _ω (t) is fed back to the microwave frequency adjustment terminal;

进行解调并进行低通滤波，获得微波功率误差信号e_b(t)，即5.2 Simultaneously with step 5.1, power modulate the frequency error signal e _ω (t)

Perform demodulation and low-pass filtering to obtain the microwave power error signal e _b (t), namely

Perform PID processing on the microwave power error signal to obtain the power control signal c _b (t), namely:

Wherein K _bp , K _bi , K _bd are PID gains;

The power control signal c _b (t) is fed back to the microwave power adjustment terminal.

A method for locking a cesium beam atomic clock, comprising the following steps:

Step 1. Modulate the microwave frequency, and set the original microwave signal as y(t)=b cos(ω ₀ t), then the modulated microwave signal is expressed as:

经过数字芯片产生并通过DAC转化为模拟信号，设对应的数字信号为

采样率为F_s；Among them, the modulated signal

It is generated by a digital chip and converted into an analog signal by a DAC. Let the corresponding digital signal be

The sampling rate is F _s ;

Step 2. Perform a microwave frequency scan:

解调并进行数字低通滤波，产生用于频率锁定的频率误差信号e_ω[n]：2.1 Feed the microwave signal modulated in step 1 into the cesium beam tube to obtain the atomic photodetection signal r(t), sample the analog signal r(t) with the sampling rate F _s by the ADC, and convert it into a digital signal r[n ], and modulate the square wave with frequency to r[n] in the FPGA

Demodulation and digital low-pass filtering produces a frequency error signal e _ω [n] for frequency locking:

2.2 Fix the microwave power b, and change the microwave frequency near the resonance point ω ₀ , that is, repeat the above step 2.1 with a predetermined frequency step size from the minimum frequency to the maximum frequency in the frequency interval on the left and right sides of ω ₀ to obtain the frequency error signal e _ω [n], find the maximum value of the frequency error signal, and record the microwave frequency as ω ₁ ;

Step 3. Scan the microwave power, set the microwave frequency at ω ₁ , change the microwave power b, that is, repeat the above step 2.1 with a predetermined power step size from the minimum power to the maximum power in the power interval on the left and right sides of b to obtain the microwave Power-frequency error signal amplitude sweep diagram, record the microwave power b ₁ when the frequency error signal e _ω [n] takes the maximum value;

Step 4. Microwave Power Modulation

The microwave power is modulated near the microwave power b ₁ point, and the modulated signal is expressed as:

为调制幅度为b_m，调制周期为T_b，均值为0的功率调制信号，采用方波进行微波率调制，

由数字芯片FPGA产生

并经过DAC产生；in,

is a power modulation signal with a modulation amplitude of b _m , a modulation period of T _b , and a mean value of 0, using a square wave for microwave rate modulation,

Generated by digital chip FPGA

And generated by DAC;

Step 5. Microwave frequency-power joint locking

5.1 Perform digital proportional-integral-derivative processing on the frequency error signal e _ω [n] to obtain the frequency control signal c _ω [n], namely:

Select the PID gains K _ωp , K _ωi , K _ωd to adjust the loop gain and bandwidth, convert the frequency control signal c _ω [n] into an analog signal c _ω (t) through the DAC and feed it back to the microwave frequency adjustment end to realize the microwave frequency closed-loop locking;

进行解调并进行数字低通滤波，滤波后的信号作为微波功率误差信号e_b[n]，即5.2 Simultaneously with step 5.1, use the power modulation signal for the frequency error signal e _ω (t)

Perform demodulation and digital low-pass filtering, and the filtered signal is taken as the microwave power error signal e _b [n], that is

PID processing is performed on the microwave power error signal, and appropriate gains K _bp , K _bi , and K _bd are selected to obtain the power control signal c _b [n], namely:

The power control signal c _b [n] is converted into an analog signal c _b (t) through the DAC and fed back to the microwave power adjustment terminal to realize the closed-loop locking of the microwave power.

Beneficial effects: Due to the influence of adjacent transition lines in the actual situation, the atomic signal will increase when the microwave power is large (see Figure 2). At this time, the traditional method of microwave power locking is prone to loss of lock; The invention uses the frequency error signal to lock the microwave power, the peak value is unique, the lock is not easily lost, and better frequency stability can be obtained.

Description of drawings

Accompanying drawing 1 is the block diagram of the locking method of cesium beam atomic clock of the present invention;

Accompanying drawing 2 is microwave power-atomic signal amplitude relation diagram;

Accompanying drawing 3 is the response curve of frequency error signal to microwave power;

Accompanying drawing 4 is the structural block diagram of magnetic selective state-light detection cesium beam atomic clock;

Figure 5 is the demodulated microwave frequency locking error signal.

Detailed ways

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings 1-5 by taking the magnetic selective state-photodetection type cesium atomic clock as an example.

Figure 4 shows the principle block diagram of the magnetically selected state-optical detection cesium beam atomic clock. There are currently three types of cesium beam atomic clocks, including magnetically selected state-electron multiplier detection, optical pumping-optical detection, and magnetically selected state-optical detection. , In this embodiment, the method for controlling the precision of a cesium beam atomic clock is described by taking the magnetic selection state-optical detection as an example, but this embodiment is not limited to this, and is also applicable to other forms of cesium beam atomic clocks.

As shown in Figure 4, atoms are ejected through a cesium furnace, and state preparation is achieved after state selection by a state selection magnet. In the process of magnetic state selection, atoms with |F=3> are usually selected. After that, the atom enters the U-shaped microwave cavity and interacts with the microwave twice to realize Ramsey interference. The microwave frequency in the U-shaped microwave cavity is generated by frequency doubling of the voltage-controlled crystal oscillator. When the microwave frequency is consistent with the atomic frequency, the atom transitions to | The probability of F=4> state is the greatest. The atoms after the transition enter the detection area, and the laser generated by the distributed feedback laser (DFB) irradiates the detection area. In the present invention, a photodetection circuit is used to detect the atoms after the transition, and the interaction between the laser and the atoms is used to make |F=4> The atoms of |F=4> and |F'=5> state cyclic transitions, resulting in fluorescence. The photodiode of the photodetection circuit can convert the fluorescence of spontaneous emission of atoms into electrical signals, which can be used as the output signal of the photodetection of the cesium beam tube.

The basic principle of the cesium beam atomic clock is to use the interaction between microwaves and atoms to realize the transition between the ground states of cesium atoms. Since the atomic transition probability is related to the microwave power, in order to ensure that the output frequency of the cesium beam atomic clock is disturbed as little as possible, it is necessary to suppress the drift of the microwave power and lock the microwave power of the cesium beam atomic clock. The existing servo locking method for microwave power is mainly based on the response of the atomic beam to different microwave powers. Modulation (eg 10 ^-2 Hz), since the microwave transition spectral line has a certain response to the microwave power, the modulation information of the microwave power will be reflected in the detection signal. After demodulating the detection signal, the error signal about the microwave power can be obtained. After the closed-loop locking, the difference between the spectral line signals under the two power points is 0, that is, the microwave power locking is realized.

However, this method has certain drawbacks in cesium beam atomic clocks. Figure 2 compares the relationship between the theoretical photodetection signal and the experimentally measured photodetection signal and microwave power. The experimental results show that the amplitude of the fluorescence signal will increase at high power. This is because at high microwave power, other lines The result of the superposition of spectra (mainly adjacent σ transition lines) at the central spectral line. This effect leads to the deformation of the microwave power response curve of the signal. When using the traditional power locking method, the locking point of the microwave power will deviate from the maximum microwave power under the ideal two-level assumption (only consider atoms with |m _F =0>). position about 1dB. In addition, due to the line shape change under high microwave power, there is a certain risk of mis-locking.

In order to lock the microwave power, the present invention proposes to use the error signal amplitude to lock the microwave power. Here, the implementation scheme of the method is described by taking the analog locking as an example.

Step 1. First, the microwave frequency needs to be modulated. Let the original microwave signal be y(t)=b cos(ω ₀ t), where b is the microwave signal amplitude, and ω ₀ is the atomic resonance frequency, which is about 2π·9192631770Hz, then The modulated microwave signal can be written as:

为幅度为ω_m，调制周期为T_ω，均值为0的频率调制信号，本发明中采用方波进行微波频率调制，即：in,

is a frequency modulation signal whose amplitude is ω _m , the modulation period is T _ω , and the mean value is 0. In the present invention, a square wave is used for microwave frequency modulation, namely:

为10⁺²Hz量级。frequency of the frequency modulated signal

on the order of 10 ⁺² Hz.

Step 2. Perform a microwave frequency scan:

解调并进行低通滤波(LPF，图1中未示出)，产生用于频率锁定的频率误差信号e_ω(t)：2.1 Feed the microwave signal modulated in step 1 into the cesium beam tube to obtain the atomic photodetection signal r(t). Modulate a square wave with frequency

Demodulation and low pass filtering (LPF, not shown in Figure 1) yields the frequency error signal e _ω (t) for frequency locking:

2.2 Fix the microwave power b, and change the microwave frequency near the resonance point ω ₀ , that is, repeat the above step 2.1 with a predetermined frequency step from the minimum frequency to the maximum frequency in the frequency interval on the left and right sides of ω ₀ , and obtain the frequency as shown in Figure 5 Scan the error signal e _ω (t) to find the maximum value of the frequency error signal, and record the microwave frequency as ω ₁ at this time.

Step 3. Scan the microwave power, set the microwave frequency at ω ₁ , and change the microwave power b, that is, repeat the above step 2.1 with a predetermined power step size from the minimum power to the maximum power in the power interval on the left and right sides of b, and obtain as follows: The microwave power-frequency error signal amplitude scanning diagram of Fig. 3, record the microwave power b ₁ when the frequency error signal e _ω (t) takes the maximum value;

Step 4. Microwave Power Modulation

The microwave power is modulated near the microwave power b ₁ point, and the modulated signal can be written as:

为幅度为b_m，调制周期为T_b，均值为0的功率调制信号，本方案中采用方波进行微波功率调制，即：in,

is a power modulation signal with an amplitude of b _m , a modulation period of T _b , and an average value of 0. In this scheme, a square wave is used for microwave power modulation, namely:

为10^-2Hz量级。In order to avoid the influence of microwave power modulation on the short-term frequency stability of atomic clocks, the modulation is usually achieved by low-frequency signals.

on the order of ^10-2 Hz.

Step 5. Microwave frequency-power joint locking

5.1 Perform proportional-integral-derivative (PID) processing on the frequency error signal e _ω (t) to obtain the frequency control signal c _ω (t), namely:

In the complex frequency domain it can be written as

The PID gains K _ωp , K _ωi and K _ωd are selected to adjust the loop gain and bandwidth, and the frequency control signal c _ω (t) is fed back to the microwave frequency adjustment terminal to realize the closed-loop locking of the microwave frequency.

进行解调并进行低通滤波(LPF，图1中未示出)，滤波后的信号作为微波功率误差信号e_b(t)，即5.2 Simultaneously with step 5.1, use the power modulation signal for the frequency error signal e _ω (t)

Demodulation and low-pass filtering (LPF, not shown in Figure 1) are performed, and the filtered signal is used as the microwave power error signal e _b (t), that is

PID processing is performed on the microwave power error signal, and appropriate gains K _bp , K _bi , and K _bd are selected to obtain the power control signal c _b (t), namely:

The power control signal c _b (t) is fed back to the microwave power adjustment terminal to realize the closed-loop locking of the microwave power, and the microwave power will be locked at the maximum point of the error signal determined by the atomic beam. According to the locking principle, assuming that the microwave power amplitude-frequency gain is A(s), then according to the control principle, the frequency domain representation of the microwave power signal noise can be written as

Among them, d(s) is the power noise of the microwave circuit itself, and η(s) is the noise of the atomic signal itself. At this time, the in-band noise is mainly determined by the atomic signal, and the power drift of the microwave circuit can be suppressed.

The relationship between the error signal detected in the experiment and the microwave power change is shown in the error signal curve in Figure 3. In this method, first, the microwave power will be locked at the position where the error signal is maximized, optimizing the short-term frequency stability (improving the Second, observing the measured curve, it can be found that the error signal will not rise monotonically so as to exceed the peak value when the microwave power is large, so the microwave power will not be wrongly locked to the position of the high microwave power. In addition, the modulation frequency for generating the error signal is increased from 1 Hz in the traditional method to 10 ² Hz, which reduces the influence of the low-frequency noise of the photodetection circuit. The contrast ratio and signal-to-noise ratio of locked microwave power are higher, which improves the performance of microwave power locking.

The present invention can also be implemented by digital locking. The following takes digital locking as an example to describe the implementation scheme of the method.

Step 1. Modulate the microwave frequency. Let the original microwave signal be y(t)=b cos(ω ₀ t), then the modulated microwave signal can be written as:

经过数字芯片，如FPGA、单片机等产生并通过DAC转化为模拟信号，设对应的数字信号为

采样率为F_s。Among them, the modulated signal

Generated by digital chips, such as FPGA, single-chip microcomputer, etc. and converted into analog signals by DAC, the corresponding digital signals are set as

The sampling rate is F _s .

Step 2. Perform a microwave frequency scan:

解调并进行数字低通滤波(LPF，图1中未示出)，产生用于频率锁定的频率误差信号e_ω[n]：2.1 Feed the microwave signal modulated in step 1 into the cesium beam tube to obtain the atomic photodetection signal r(t), sample the analog signal r(t) with the sampling rate F _s by the ADC, and convert it into a digital signal r[n ], and modulate the square wave with frequency to r[n] in the FPGA

Demodulation and digital low pass filtering (LPF, not shown in Figure 1) yields the frequency error signal e _ω [n] for frequency locking:

2.2 Fix the microwave power b, and change the microwave frequency near the resonance point ω ₀ , that is, repeat the above step 2.1 with a predetermined frequency step size from the minimum frequency to the maximum frequency in the frequency interval on the left and right sides of ω ₀ to obtain the frequency error signal e _ω [n] Scan the graph to find the maximum value of the frequency error signal, and record the microwave frequency as ω ₁ at this time.

Step 3. Scan the microwave power, set the microwave frequency at ω ₁ , and change the microwave power b, that is, repeat the above step 2.1 with a predetermined power step size from the minimum power to the maximum power in the power interval on the left and right sides of b, and obtain as follows: The microwave power-frequency error signal amplitude scanning diagram of Fig. 3, record the microwave power b ₁ when the frequency error signal e _ω [n] takes the maximum value;

Step 4. Microwave Power Modulation

The microwave power is modulated near the microwave power b ₁ point, and the modulated signal can be written as:

为调制幅度为b_m，调制周期为T_b，均值为0的功率调制信号，本方案中采用方波进行微波功率调制，

由数字芯片FPGA产生

并经过DAC产生。in,

is a power modulation signal with a modulation amplitude of b _m , a modulation period of T _b , and an average value of 0. In this scheme, a square wave is used for microwave power modulation,

Generated by digital chip FPGA

And generated by DAC.

Step 5. Microwave frequency-power joint locking

5.1 Perform digital proportional-integral-derivative (PID) processing on the frequency error signal e _ω [n] to obtain the frequency control signal c _ω [n], namely:

Select the PID gains K _ωp , K _ωi , K _ωd to adjust the loop gain and bandwidth, convert the frequency control signal c _ω [n] into an analog signal c _ω (t) through the DAC and feed it back to the microwave frequency adjustment end to realize the microwave frequency closed-loop locking.

进行解调并进行数字低通滤波(LPF，图1中未示出)，滤波后的信号作为微波功率误差信号e_b[n]，即5.2 Simultaneously with step 5.1, use the power modulation signal for the frequency error signal e _ω (t)

Demodulation and digital low-pass filtering (LPF, not shown in Figure 1) are performed, and the filtered signal is taken as the microwave power error signal e _b [n], i.e.

PID processing is performed on the microwave power error signal, and appropriate gains K _bp , K _bi , and K _bd are selected to obtain the power control signal c _b [n], namely:

The power control signal c _b [n] is converted into an analog signal c _b (t) through the DAC and fed back to the microwave power adjustment terminal to realize the closed-loop locking of the microwave power, and the microwave power will be locked at the maximum point of the error signal determined by the atomic beam. .

Finally, it should be noted that there are various implementations of the microwave power locking scheme used in the present invention. Locking period, modulation frequency, analog/digital locking scheme, etc. cannot constitute limitations of the present invention.

Claims (2)

1. A locking method of a cesium beam atomic clock is characterized by comprising the following steps: step 1, modulating microwave frequency; step 2, microwave frequency scanning is carried out to obtain a frequency error signal of frequency locking; step 3, scanning the microwave power; step 4, modulating microwave power; and 5, locking the microwave frequency-power combination.

2. The method for locking cesium beam atomic clocks according to claim 1, characterized by step 1 comprising:

let the original microwave signal be y (t) ═ b cos (ω)₀t), where b is the microwave signal amplitude, ω₀For atomic resonance frequency, the modulated microwave signal is expressed as:

wherein,

modulating amplitude to omega for microwave frequency_mModulation period of T_ωFrequency modulation signal with average value of 0; adopting square wave to perform microwave frequency modulation, namely:

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